Exotic Decay Channels Research Articles

Hunting for top partner with a new signature at the LHC

A vector-like top partner plays a central role in many new physics models that attempt to address the hierarchy problem. The top partner is conventionally assumed to decay to a quark and a Standard Model (SM) boson. However, it is also possible that the top partner decays to a non-SM scalar and a top quark. Such an exotic decay channel can be the main decay channel of top partners, and thus provides new windows to search for top partners at the LHC. In this paper, with the classical machine learning method of boosted decision trees, we perform a model-independent study of the discovery potential of this new signature at the LHC. In order to suppress the main QCD background, we consider the subdominant but clean decay channel $a\ensuremath{\rightarrow}\ensuremath{\gamma}\ensuremath{\gamma}$. For completeness, the single production process and pair production process of top partners are taken into account. We find that, for both single and pair production, the future high-luminosity LHC can exclude a top partner mass up to the TeV scale through the channel $T\ensuremath{\rightarrow}ta$ ($a\ensuremath{\rightarrow}\ensuremath{\gamma}\ensuremath{\gamma}$), even if $\mathrm{BR}(a\ensuremath{\rightarrow}\ensuremath{\gamma}\ensuremath{\gamma})$ is as small as $\mathcal{O}(0.1%)$. Besides, our result shows that single production can overmatch pair production at the 14 TeV LHC, provided that the top partner is heavier than 800--900 GeV.

Probing exotic charged Higgs decays in the Type-II 2HDM through top rich signal at a future 100 TeV pp collider

The exotic decay modes of non-Standard Model Higgs bosons are efficient in probing the hierarchical Two Higgs Doublet Models (2HDM). In particular, the decay mode H±→HW± serves as a powerful channel in searching for charged Higgses. In this paper, we analyze the reach for H±→HW±→ toverline{t}W at a 100 TeV pp collider, and show that it extends the reach of the previously studied ττW final states once above the top threshold. Top tagging technique is used, in combination with a boosted decision tree classifier. At the low tan β region, almost the entire hierarchical Type-II 2HDM parameter space can be probed via the combination of all exotic decay channels.

Neutron decay to a non-Abelian dark sector

According to the Standard Model (SM), we expect to find a proton for each decaying neutron. However, the experiments counting the number of decayed neutrons and produced protons disagree. This discrepancy suggests that neutrons might have an exotic decay channel to Dark Sector (DS) particles. In this paper, we explore a scenario where neutrons decay to a dark Dirac fermion $\ensuremath{\chi}$ and a non-Abelian dark gauge boson ${W}^{\ensuremath{'}}$. In our proposed scenario, the DS has three portals with the SM sector: (1) the fermion portal coming from the mixing of the neutron with $\ensuremath{\chi}$, (2) a scalar portal, and (3) a nonrenormalizable kinetic mixing between photon and dark gauge bosons which induces a vector portal between the two sectors. We discuss the cosmological implications of this scenario assuming DS particles are produced via freeze-in. The fermion and the scalar portal leads to the overproduction of DS particles by the time of the Cosmic Microwave Background (CMB), and thus we disable these two portals in the early universe. For that, we require the maximum temperature of the universe to be lower than ${m}_{\ensuremath{\chi}}$. We rely on the vector portal to connect the two sectors in the early universe, and we discuss the phenomenological bounds on the model. The main constraints come from the Big Bang Nucleosynthesis, ensuring the right relic abundance of dark matter, and the observation of large neutron stars.

2HDM neutral scalars under the LHC

Two Higgs Doublet Models (2HDM) provide a simple framework for new physics models with an extended Higgs sector. The current LHC results, including both direct searches for additional non-Standard Model (SM) Higgs bosons, as well as precision measurements of the SM-like Higgs couplings, already provide strong constraints on the 2HDM parameter spaces. In this paper, we examine those constraints for the neutral scalars in the Type-I and Type-II 2HDM. In addition to the direct search channels with SM final states: H/A → foverline{f} , VV, Vh, hh, we study in particular the exotic decay channels of H/A → AZ/HZ once there is a mass hierarchy between the non-SM Higgses. We found that H/A → AZ/H Z channel has unique sensitivity to the alignment limit region which remains unconstrained by conventional searches and Higgs precision measurements. This mode also extends the reach at intermediate tβ for heavy mA that are not covered by the other direct searches.

Higgs boson decay into four bottom quarks in the SM and beyond

We present predictions for the Higgs boson decay into four bottom quarks in the standard model and via light exotic scalars retaining full bottom-quark mass dependence. In the SM the decay can be induced either by the Yukawa couplings of bottom quarks and top quarks or the electroweak couplings. We calculate the partial decay width and various differential distributions up to next-to-leading order in QCD. We find large QCD corrections for decay via Yukawa couplings, as large as 90% for the partial decay width, and reduced scale variations. The results of this paper are therefore helpful for the measurement of this multi-jets final state at future Higgs factory of electron-positron colliders. We also propose several observables that can differentiate the SM decay channel and the exotic decay channel and compare their next-to-leading order predictions.

Exotic Higgs decays in Type-II 2HDMs at the LHC and future 100 TeV hadron colliders

The exotic decay modes of non-Standard Model (SM) Higgses in models with extended Higgs sectors have the potential to serve as powerful search channels to explore the space of Two-Higgs Doublet Models (2HDMs). Once kinematically allowed, heavy Higgses could decay into pairs of light non-SM Higgses, or a non-SM Higgs and a SM gauge boson, with branching fractions that quickly dominate those of the conventional decay modes to SM particles. In this study, we focus on the prospects of probing Type-II 2HDMs at the LHC and a future 100 TeV pp collider via exotic decay channels. We study the three prominent exotic decay channels: A → HZ, A → H±W∓ and H± → HW±, and find that a 100-TeV pp collider can probe most of the region of the Type-II 2HDM parameter space that survives current theoretical and experimental constraints with sizable exotic decay branching fraction through these channels, making them complementary to the conventional decay channels for heavy non-SM Higgses.

Study of spin-isospin responses of radioactive nuclei with the background-reduced neutron spectrometer, PANDORA

The status of a project to measure spin-isospin responses of neutron drip-line nuclei using a new low-energy neutron detector, PANDORA (Particle Analyzer Neutron Detector Of Real-time Acquisition), is reported. The performance of PANDORA was characterized by the 6He(p,n)6Li reaction in inverse kinematics at the HIMAC facility in Chiba. Observation of the strong transition to the ground state in 6Li is discussed. Preliminary results of 11Li(p,n)11Be and 14Be(p,n)14B experiments in inverse kinematics at RI Beam Factory (RIBF) of RIKEN Nishina Center are also presented including the exotic decay channel of 11Be→9Li + d. Details of the experimental setup based on PANDORA and the SAMURAI large-acceptance magnetic spectrometer, as well as the combined data-acquisition system are described. The neutron-gamma discrimination capability of PANDORA was evaluated, Figure-of-Merit (FoM) values higher than those found in the literature for similar materials were derived from experimental data.

Exotic decay channels are not the cause of the neutron lifetime anomaly

Since long neutron lifetimes measured with a beam of cold neutrons are significantly different from lifetimes measured with ultracold neutrons bottled in a trap. It is often speculated that this “neutron anomaly” is due to an exotic dark neutron decay channel of unknown origin. We show that this explanation of the neutron anomaly can be excluded with a high level of confidence when use is made of our new result for the neutron decay β asymmetry. Furthermore, data from neutron decay now compare well with Ft-data derived from nuclear β decays.

Common exotic decays of top partners

Many standard model extensions that address the hierarchy problem contain Dirac-fermion partners of the top quark, which are typically expected around the TeV scale. Searches for these vector-like quarks mostly focus on their decay into electroweak gauge bosons and Higgs plus a standard model quark. In this article, backed by models of composite Higgs, we propose a set of simplified scenarios, with effective Lagrangians and benchmarks, that include more exotic decay channels, which modify the search strategies and affect the bounds. Analysing several classes of underlying models, we show that exotic decays are the norm and commonly appear with significant rates. All these models contain light new scalars that couple to top partners with charge 5/3, 2/3, and −1/3.

Review of fundamental nuclear-geophysical researches in OSENU: i. standard beta decay theory elements and new cooperative electron-β-nuclear spectroscopy

This paper opens a series of publications, devoted to review of new results of the original nuclear-geophysical researches at the OSENU mathematics department and its laboratories (computational and quantum mathematics and mechanics, environmental radioactivity, nuclear geophysics). Primarily the first material will focus on fundamental issues that constitute the theoretical basis of the further applied nuclear-geophysical research. It is shortly presented a modern approach to computing the key parameters of the beta decay processes. There are discussed the cooperative electron β-nuclear processes in atoms and molecules, including the excitation, ionization, electronic rearrangement, induced by the nuclear reactions and β-decay. A few factors are taken into account: changing the integration limits in the Fermi function integral, energy corrections for different chemical substances, and the possibility of the bound β-decay or other exotic decay channels.

Indirect detection imprint of aCPviolating dark sector

We introduce a simple scenario involving fermionic dark matter ($\ensuremath{\chi}$) and singlet scalar mediators that may account for the Galactic center GeV $\ensuremath{\gamma}$-ray excess while satisfying present direct detection constraints. $CP$ violation in the scalar potential leads to a mixing between the Standard Model Higgs boson and the scalar singlet, resulting in three scalars, ${h}_{1,2,3}$, of indefinite $CP$-transformation properties. This mixing enables $s$-wave $\ensuremath{\chi}\overline{\ensuremath{\chi}}$ annihilation into discalar states, followed by decays into four-fermion final states. The observed $\ensuremath{\gamma}$-ray spectrum can be fitted while respecting present direct detection bounds and Higgs boson properties for ${m}_{\ensuremath{\chi}}=60\ensuremath{\sim}80\text{ }\mathrm{GeV}$, and ${m}_{{h}_{3}}\ensuremath{\sim}{m}_{\ensuremath{\chi}}$. Searches for the Higgs exotic decay channel ${h}_{1}\ensuremath{\rightarrow}{h}_{3}{h}_{3}$ at the 14 TeV LHC should be able to further probe the parameter region favored by the $\ensuremath{\gamma}$-ray excess.

Light charged Higgs bosons to AW/HW via top decay

While current ATLAS and CMS measurements exclude a light charged Higgs ($m_{H^\pm}<160$ GeV) for most of the parameter region in the context of the MSSM scenarios, these bounds are significantly weakened in the Type II 2HDM once the exotic decay channel into a lighter neutral Higgs, $H^\pm \to AW/HW$, is open. In this study, we examine the possibility of a light charged Higgs produced in top decay via single top or top pair production, with the subsequent decay $H^\pm \rightarrow AW/HW$, which can reach a sizable branching fraction at low $\tan\beta$ once it is kinematically permitted. With a detailed collider analysis, we obtain exclusion and discovery bounds for the 14 TeV LHC assuming the existence of a 70 GeV neutral scalar. Assuming ${\rm BR}(H^\pm \rightarrow AW/HW)=100\%$ and ${\rm BR}(A/H \rightarrow \tau\tau)=8.6\%$, the 95% exclusion limits on ${\rm BR}(t \rightarrow H^+ b)$ are about 0.2% and 0.03% for single top and top pair production respectively, with an integrated luminosity of 300 ${\rm fb}^{-1}$. The discovery reaches are about 3 times higher. In the context of the Type II 2HDM, discovery is possible at both large $\tan\beta > 17$ for 155 GeV $< m_{H^\pm} <$ 165 GeV, and small $\tan\beta < 6$ over the entire mass range. Exclusion is possible in the entire $\tan\beta$ versus $m_{H^\pm}$ plane except for charged Higgs masses close to the top threshold. The exotic decay channel $H^\pm \to AW/HW$ is therefore complementary to the conventional $H^\pm \rightarrow \tau\nu$ channel.

Supersymmetric subelectroweak scale dark matter, the Galactic Center gamma-ray excess, and exotic decays of the 125 GeV Higgs boson

We continue our exploration of the nearly Peccei-Quinn symmetric limit shared by common singlet extensions of the Minimal Supersymmetric Standard Model. This limit has been established as a viable framework for studying sub-electroweak scale dark matter phenomenology and has interesting and direct connections to new exotic Higgs decay physics. We present analytic calculations to motivate the important phenomenological features mentioned above. We also discuss benchmark points in this model framework that accommodate the observed Galactic Center gamma ray excess. We emphasize connections between phenomenology of dark matter direct detection and indirect detection, and new exotic decay channels for the 125 GeV Higgs boson. We conclude by identifying two benchmark modes of exotic Higgs decays for $h \to \tau^+ \tau^- \mbox{${\not\! E}_{\rm T}$}$ and $h \to b \bar{b} \mbox{${\not\! E}_{\rm T}$}$ final states and estimate their sensitivity prospects at the LHC.